The term "aramid" is used to designate wholly aromatic polyamides. Not all aramid fibers are useful in the present invention but only those derived from aromatic polyamides whose chain extending bonds are either coaxial or parallel and oppositely directed. High strength, high modulus aramid fibers useful in the present invention may be prepared by the processes described in U.S. Pat. Nos. 3,767,756 and 3,869,430. The fibers are characterized by filament tenacities of at least 18 gpd (15.9 dN/tex) and moduli of at least 400 gpd (354 dN/tex). These fibers will be referred to hereinafter as p-aramid fibers. Particularly preferred are p-aramid fibers based on poly(p-phenylene terephthalamide) as produced by Du Pont under the trademark Kevlar.RTM..
P-aramid fibers are characterized by their excellent high-temperature durability. Not only do such fibers fail to soften at temperatures which would melt and destroy ordinary fibers, but in general, they have no melting point. Thus, such fibers cannot be shaped from a polymer melt as in the case of the nylons and the polyesters but rather they are shaped from polymer solutions. In general, it is more economical to wet spin or dry spin polymer solutions to produce a single large bundle of filaments than it is to spin the same number of filaments into several smaller bundles. However, a need exists for the smaller yarns.
An alternative to the spinning of smaller bundles of continuous filaments is to cut filaments of a heavy denier tow into staple fibers and process the staple fibers into spun yarns of the desired fineness. A sacrifice in yarn strength is an expected drawback in this alternative, which becomes more important as the size of the staple yarn decreases. As a general rule of thumb, an average of at least 60 synthetic or 80 cotton fibers per yarn cross section is needed for yarns of minimum acceptable strength at reasonable levels of twist. Lower than this number of fibers per cross section provides less than the minimum amount of fiber-to-fiber interaction required for good yarn strength. It is well known that synthetic filaments can be cut or stretch-broken to produce staple or sliver, respectively, and that either can be converted to useful staple-spun yarns [via e.g., the cotton or worsted systems].
It has now been found that p-aramid continuous-filament yarn or tow stretch broken on a Turbo Stapler or equivalent yields unexpectedly strong spun yarns when processed to spun yarns in a conventional manner.